22 INDEX TO THE STEATIGRAPHY OF NOKTH AMERICA. 



In order to discuss contrasts it is necessary to have in mind certain facts about 

 colors. There are three primary colors — red, blue, and yellow, which may be more 

 accurately spoken of in terms of pigments as crimson lake, cobalt blue, and cad- 

 mium yellow. If pairs of these colors are combined in equal volumes they yield 

 the standard secondary colors purple, orange, and green. Again if the standard 

 secondaries are mixed in equal volumes in pairs standard tertiary colors are 

 obtained — russet from purple and orange; olive from purple and green; and citrine 

 from orange and green. Red predominates in russet, blue in olive, and yellow 

 in citrine; each primary forms one-half of the respective tertiary color, but the 

 presence of the other two primaries modifies its brilliancy. 



Although the above statements are true in a general way, they are not mathe- 

 matically exact. A name does not define a color. There are various reds, blues, 

 and yellows, and various secondaries and tertiaries result according to the choice 

 of primaries with which to start. Given three primaries, however, corresponding 

 standard secondaries and tertiaries may be produced in the rnanner above described. 



If graduated proportions are mixed instead of equal parts a series of slightly 

 differing tints are obtained. For instance, 9 volumes of red to 1 of blue, 8 of red 

 to 2 of blue, 7 of red to 3 of blue, and so on to 1 of red to 9 of blue will yield a range 

 of purples which are scarcely distinguishable one from the next, yet which run the 

 gamut from red to blue. A similar range of greens from blue to yellow and one 

 of oranges from yellow to red complete a circle of closely graded secondaries. 



The tertiary mixtures may be graded in the same manner to produce a 

 sequence of russets, olives, and citrines which range from purple through green 

 and then through orange back to purple without a break. 



If we call the primary, secondary, and tertiary colors thus mixed hues, we 

 may distinguish tints and shades of any one of them. The normal hue has the full 

 available strength of its constituent colors. A tint is lighter. It may be mixed 

 by adding more of the medium, water or oil, or by adding white. On the other 

 hand a shade is darker — that is, it reflects less light than the normal hue. It may 

 be produced by adding black, but more artistically by mixing a complementary 

 tertiary color with the normal hue. In color printing tints result where the white 

 of the paper shines through the hue, which is thereby diluted, and shades are 

 produced by overprinting grays, which are usually tints of olives or russets strong 

 in blue. 



Contrast depends on the juxtaposition of unlike hues. It is strongest between 

 any two of the pure primaries. It is equally striking but less likely to be offensive 

 between a primary and the secondary which is composed of the other two primaries. 

 Contrast in general is less pronounced between secondary hues and is materially 

 less between tertiaries. There is no stronger contrast than that of white and black; 

 in relation to colors it enters in a subdued degree into the distinction between tints 

 and shades. Hence where gtrong contrast is desired we may juxtapose primary 

 hues, or a primary with its complementary secondary, or a light tint of a hue against 

 a dark shade of the same or another hue. The requirement of distinctness may 

 usually be met by lesser contrasts and in general the greater contrasts are reserved 

 for small areas or for emphasis. 



